Touch sensors are used in many electronic and computing devices. Many laptops, for example, are equipped with a rectangular touch sensor that functions like a computer mouse to control pointer positioning on a screen and permit entry of commands. The touch sensor detects the user's touch and generates signals representing a location of contact on the sensor.
FIG. 1 shows a conventional linear touch sensor 100. For discussion purposes, the sensor 100 is illustrated with four rectangular sensor pads 102(1), 102(2), 102(3), and 102(4) that are linearly aligned. This arrangement allows detection of a user's touch within four discrete zones 1-4 that correspond with the four sensor pads 102(1)-102(4).
To increase precision, additional detection zones may be created by changing the shape of the sensor pads. FIG. 2 shows another conventional linear touch sensor 200 having five discrete sensor pads 202(1)-202(5), where each sensor pad has a non-rectangular shape. Edges between adjacent sensor pads are discontinuous or jagged. This shape allows adjoining sensor pads to interlace with one another to define additional detection zones, so that there are more zones than sensor pads. In this example, there are nine detection zones 1-9 for five sensor pads. With this arrangement, a single touch in zone 2 is detected by adjacent sensor pads 202(1) and 202(2), whereas a touch in zone 1 is detected solely by the top sensor pad 202(1).
Many devices today use non-rectangular touch sensors. For instance, some popular audio devices (e.g., MP3 players) employ circular touch sensors. These sensors have traditionally followed the same design as linear sensors, with multiple discrete sensor pads (e.g., 12-16 sensors) aligned side-by-side. FIG. 3 shows a conventional circular sensor 300 having sixteen discrete sensor pads 302 extending radially outward from the center. To increase precision, the edges between the sensor pads may be made jagged. FIG. 4 shows one example circular sensor 400 having multiple sensor pads 402 with jagged edges. Sixteen pads 402 are shown, but there may be fewer (e.g., 12).
These conventional sensors detect presence or absence of a finger on each sensor pad. Past solutions to increase precision have been to increase the number of sensor pads or make the edges jagged to define extra detection zones. However, larger numbers of sensor pads requires more expensive and complex interfaces to convert the detection signals to a smaller number of output pins on circuit chips. Moreover, even as the number of sensor pads increase the sensors still detect only a finite number of contact positions.
In addition to precision, manufacturing cost is another important consideration for designers of touch sensors. These designers continually look for ways to reduce cost. Conventional circular touch sensors employ anywhere from 12 to 16 sensors, thereby increasing component costs.
Accordingly, there remains a need for an improved circular touch sensor that is inexpensive to produce, yet provides high accuracy and precision similar to that of conventional 16-pad sensors.